The current value of the debugging flags. May be read or set. Like its command-line equivalent, you can use numeric or symbolic values, e.g.
$^D = 10 or
$^D = "st". See "-Dnumber" in perlrun. The contents of this variable also affects the debugger operation. See "Debugger Internals" in perldebguts.
Mnemonic: value of -D switch.